GASTROPODA. 1025 
Euomphalide. ] 
Euomphalus began in Ophileta. Typically, this genus is probably restricted to rocks 
of the age of the Calciferous, and consists of discoidal shells, with the spire concave 
and the base nearly flat. The whorls are narrow, in contact, all exposed, and flat- 
tened or gently convex beneath, angular at the lower part of the slightly convex or 
flattened periphery and more sharply angular on the upper side. The anterior 
extremity of the upper keel marks the bottom of a deep >-shaped apertural sinus 
or notch. The keel itself is homologous with the slit-band of the Plewrotomariude 
though of more simple structure. The outer lip extends forward a trifle farther or 
about as far as the innermost part of the upper, while the lower on the whole is 
nearly direct though slightly insinuated in the outer half. 
From the typical forms of the genus like 0. complanata we pass by rather easy 
gradations through forms like 0. ? bella Billings and Huomphalus uniangulatus Hall, 
to Helicotoma, Salter, the change consisting in the raising of the spire till it projected 
slightly above the level of the last volution, in the deepening of the umbilicus and in 
the rounding of the outer and lower sides of the whorls. The course of the lines of 
growth and the >-shaped notch at the end of the upper carina remain about the 
same. Butit is not from fully developed Helicotoma, which continued as an inde- 
pendent genus to the close of the Lower Silurian, that the later Euomphali were 
evolved. In our opinion they were derived from a second branch of the Calciferous 
uniangulatus section of Ophileta, the first resulting in Helicotoma, the farther develop- 
ment of which tended toward the early pleurotomarian rather than the euomphaloid 
type of structure. In Helicotoma, namely, the upper carina becomes very much like 
a slit-band, and that is precisely the opposite of what was necessary to produce an 
Euomphalus, in which the apertural notch is reduced to the minimum. 
Now, if the student will compare species like O. bella Billings, 0. (Eu.) uniangu- 
latus Hall, Eu. obtusangulus Lindstrom, E. precursor Lindstrom and E. walmstedi 
Lindstrém, the last two Upper Silurian species, we think he will be prepared to 
admit the correctness of our views, since with our present knowledge it is quite out 
of the question to arrange the last of the species mentioned in any other position 
than in the immediate vicinity of the original Carboniferous types of Hwomphalus. 
During Devonian times a side branch from Euomphalus, for the best species of 
which Hall proposed the generic name Plewronotus, became very abundant in 
individuals if not in species also. Itis interesting to note that Plewronotus represents 
a very striking return to characters pertaining to Ophileta, there being the same flat 
base, carinate upper side, and deep apertural notch which previously had been the 
main characteristics of that primitive genus. It might be contended that Ophileta 
enjoyed a continuous existence from the Calciferous to the Devonian and that 
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